1. Field of the Invention
The present invention relates to an image forming apparatus and a method of synchronizing rotations of image carriers, and in particular, an image forming apparatus including a plurality of photosensitive drums and a method of synchronizing rotations of photosensitive drums.
2. Description of the Related Art
An image forming apparatus is provided with developing members for respective colors of yellow, magenta, cyan, and black. A so-called tandem technique is employed, in which toner images in the respective colors are formed by those developing members and transferred onto a transfer belt by being overlapping with one another to thereby form a full-color image.
However, sometimes during operation the image carriers (e.g. photosensitive drums) may rotate with eccentricities (i.e. mutually-differing positions off-center) due to an operational error, for example. In that case, when the toner images are transferred onto the transfer belt by being overlapped with one another, misregistration occurs among the respective colors, which makes it difficult to form an appropriate image.
As described above, in conventional technology, in order to reduce displacement of each color due to eccentricities of each photosensitive drum, a phase mark is added to a smallest-radius direction position of each of the photosensitive drums. Then, the photosensitive drums are successively caused to stop rotating at timings at each of which a sensor detects that the phase mark has reached an uppermost direction. This allows a phase alignment of the plurality of photosensitive drums.
However, in such related art as described above, when the phase alignment is performed on each photosensitive drum, it is necessary to successively cause the photosensitive drums to stop rotating when each phase mark reaches the uppermost direction. Thus, the phase alignment may take much time using such an approach.